A uniform voltage is applied to loads even to other than a multiple of three loads made of heating lamps connected in parallel to each other, without causing any burden on a primary side. The number of loads is divided into a multiple of three and a multiple of two. A three-phase AC voltage is applied to three primary windings, which are connected as a Y-connection or a Δ-connection. Secondary windings, which are each magnetically coupled to a primary winding of a plurality of three-phase transformers and have the same number of turns, are connected with the multiple of three loads.